Verovio Humdrum Viewer
Verovio Humdrum Viewer is an online musical data editor. The primary function of the website is for viewing and editing music in the Humdrum data format, but also music can be loaded and edited in the MusicXML, MEI, MuseData, and EsAC data formats.
The VHV editor also has built-in musical data which can be accessed through the "Scores" menu. Here is an example of a score loaded from the Mozart piano sonatas entry in the menu:
This particular score comes from a Github repository for the Mozart sonatas and is loaded directly in to the VHV editor.